Ultraviolet detector

ABSTRACT

An ultraviolet detector comprises a metal tubular member which hermetically encloses an anode and a cathode therein and is filled with a discharged gas introduced therein from a metal exhaust tube. After the anode and the cathode are enclosed within the tubular member, the ultraviolet detector can be made without being subjected to any glass fusing process. Accordingly, the inside of the sealed vessel V 1  can be prevented from being contaminated with fluorine, whereby the ultraviolet detector with stable characteristics can be provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ultraviolet detector which detectsultraviolet radiation incident thereon by converting them into anelectric signal.

2. Related Background Art

An example of conventional ultraviolet detectors is disclosed inJapanese Utility model Publication No. 49-17184. This publicationdiscloses an ultraviolet detector in which an anode and a cathode aredisposed within a sealed vessel constituted by a glass envelope and aglass bottom plate welded to the bottom portion of the glass envelope.

Though the conventional ultraviolet detector mentioned above is anexcellent detector which has a long life and can stably detectultraviolet radiation, its characteristics may not be sufficient.Specifically, when used for a long period of time, its characteristicsmay deteriorate over time, thus lacking in stability.

SUMMARY OF THE INVENTION

In order to overcome such shortcomings, various studies haveconventionally been made. The inventors have elucidated that theseshortcomings result from the glass material used as a window materialfor the ultraviolet detector. Typical glass materials which aretransparent to ultraviolet radiation contain fluorine. Upon welding ofthe envelope and bottom plate of the ultraviolet detector, fluorinecontained in the glass material evaporated from the glass material andadsorbed onto the surfaces of the anode and cathode, the inner surfaceof the sealed vessel, and the like. Normal operation of the detector andthe aging process in fabrication both include the gas discharge betweenthe electrodes. Electrons and ions generated by the gas dischargeimpinge onto the surfaces of the anode and cathode respectively. Itcauses the desorption of fluorine adsorbed on the surface of theseelectrodes. The fluorine containments on the other sites in the vesselcan also be desorbed by means of the heat which arises in the agingprocesses of the detector fabrication and even in the normal operationcondition of the detector. The desorbed fluorine alters the ionizationproperty of the discharged gas filled in the vessel. This alternationcommonly results in the lowering of the breakdown voltage and that leadsto occasional and continuous false discharges and unwanted increase ofthe sensitivity. These effects considerably degrade the stability andthe reliability of the detector.

In order to overcome the foregoing shortcomings resulting from the useof such a glass material, it is an object of the present invention toprovide an ultraviolet detector having characteristics which are betterthan those conventionally attained.

The ultraviolet detector in accordance with the present inventioncomprises a sealed vessel, an anode, a cathode, a lead pin and a gasenclosed in the sealed vessel. The sealed vessel includes a tubularmember having an opening and being made of a metal material blockingultraviolet radiation, a window member being made of a glass materialtransparent to ultraviolet radiation and closing aforementioned openingand a stem having a metal portion contacting to the tubular member and aglass portion not contacting the tubular member. The anode is disposedwithin the sealed vessel at positions opposing said window member by thelead pin which penetrates the glass portion of the stem for supplyingvoltage. The cathode is disposed within the sealed vessel between thewindow member and the anode and secured to the tubular member or themetal portion of the stem.

In such a configuration, since the tubular member is made of a metalmaterial blocking ultraviolet radiation, incident ultraviolet radiationare introduced through the window member made of anultraviolet-transparent material toward the anode and cathode of thedetector, whereby the detector exhibits a high directivity. Further,since the tubular member is made of a metal material, even when thistubular member is connected to the metal portion of the stem by pressureor welding, impurities such as fluorine do not attach to the sealedvessel, anode, and cathode. Accordingly, the ultraviolet detector inaccordance with the present invention is prevented from being affectedby fluorine or the like, whereby the break down voltage of the detectorcan be held stably.

And more, the cathode of the present invention is secured to the tubularmember or the metal portion of the stem without a stem pin. So it iseasy to manufacture the ultraviolet detector having discharging gap witha high precision.

According to the present invention, the cathode may be integrated withthe tubular member or the metal portion of the stem may be a ring shapedrim of the stem.

Such configuration aids in facilitating manufacture of high accurateultraviolet detector.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing an ultraviolet detector in accordance witha first embodiment of the present invention;

FIG. 2 is a sectional view of the ultraviolet detector taken along lineII—II of FIG. 1;

FIG. 3 is a circuit diagram showing a driving circuit of the ultravioletdetector shown in FIG. 1;

FIG. 4 is a plan view showing an ultraviolet detector in accordance witha second embodiment of the present invention;

FIG. 5 is a sectional view of the ultraviolet detector taken along lineV—V of FIG. 4; and

FIGS. 6 to 11 are vertical sectional views of ultraviolet detectors inaccordance with other embodiments of the present invention,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the ultraviolet detector will beexplained. Elements identical to each other will be referred to withmarks identical to each other, without their overlapping explanationsbeing repeated. In the following explanation, vertical orientationsconform to those in the drawings.

FIG. 1 is a plan view of an ultraviolet detector D1 in accordance with afirst embodiment of the present invention. FIG. 2 is a sectional view ofthe ultraviolet detector D1 taken along line II—II of FIG. 1. Thisdetector comprises a sealed vessel V1, and an anode 1 and a cathode 2which are disposed within the sealed vessel V1.

The sealed vessel V1 comprises a tubular member 3, made of a metalmaterial blocking ultraviolet radiation, having two openings; a windowmember 4, made of an ultraviolet-transparent glass material, closing oneof the openings of the tubular member 3; a ring-shaped metal member 5secured to the tubular member 3 so as to close the other opening of thetubular member 3; and a glass sealant 7 sealing the opening in thering-shaped metal member 5. The lower side wall portions of the tubularmember 3 and ring-shaped metal member 5 are curved so as to projectoutward, and their curved portions are electrically welded together soas to overlap each other. The middle side wall portion of thering-shaped metal member 5 is in parallel with the middle side wallportion of the tubular member 3, thus constituting a cylinder. The upperside wall portion of the ring-shaped metal member 5 is curved inward,and this upper curved portion has an outer surface 5a which is used forpositioning the anode 1.

The region of the anode 1 opposing the window member 4 is depressed,with respect to its surrounding area, toward the cathode 2. Also, a gridor mesh 1m is formed in this region. The anode 1 extends from thesurrounding area of the depression toward the positioning outer surface5a of the ring-shaped metal member 5, and its end portion 1a in theextending direction is curved outward so as to be in parallel with theouter surface 5a of the upper end of the ring-shaped metal member 5. Theanode 1 is positioned with respect to the ring-shaped member 5 when itsend portion 1a is simply fixed with respect to the outer surface 5a.

The cathode 2 is placed at a position opposing the mesh region 1m formedat the depression of the anode 1. From the lower surface of the cathode2, a lead pin 6 extends through the center of the ring-shaped metalmember 5. The lead pin 6 is firmly embedded in the glass sealant 7filling the opening of the ring-shaped metal member 5. Accordingly, theanode 1 is positioned with respect to the cathode 2 connected to thelead pin 6 when the end portion la is simply fixed with respect to theouter surface 5a of the ring-shaped metal member 5. Also embedded in theglass sealant 7 is a metal evacuation pipe 8 communicating with theinside of the sealed vessel V1. The metal evacuation pipe 8 is used forintroducing a rare gas such as argon into the sealed vessel V1. Aftersuch a gas is introduced, the outer end of the metal evacuation pipe 8is sealed. For the cathode 2, any material can be used as long as it hasa work function of 4.1 eV or higher. For example, Ni (nickel), Mo(molybdenum), or W (tungsten) may be used. The material for the cathode2 in this embodiment is Ni, whereas the lead pin 6 and the tubularmember 3 are made of covar. The window member 4 is made ofultraviolet-transparent glass (UV glass), and ultraviolet radiationhaving a wavelength of about 190 nm or longer can be transmittedtherethrough. In the case where the UV glass is made ofultraviolet-transparent borosilicate glass, its coefficient of thermalexpansion can be made closer to that of covar metal, whereby it can beeasily attached to the tubular member 3, thus facilitating themanufacture of the ultraviolet detector.

FIG. 3 is a circuit diagram showing a driving circuit of the ultravioletdetector D1. When a voltage is applied between the tubular member 3 andthe lead pin 6 from a power supply S1 by way of resistors R1 and R2, thevoltage is applied between the anode 1 and the cathode 2, therebygenerating an electric field. The applied voltage is higher than thelowest voltage that discharges between the anode 1 and cathode 2 can beinduced in response to incident ultraviolet radiation, while being lowerthan the lowest voltage that spontaneously induces discharge when thereis no incident ultraviolet radiation. In this embodiment, a voltage ofabout 350 V is applied. Since the tubular member 3 is made of a metalmaterial blocking ultraviolet radiation, incident ultraviolet radiationare introduced toward the anode 1 and cathode 2 of the detector D1through the window material 4 made of an ultraviolet-transparentmaterial. Accordingly, the detector D1 has a high directivity. In thisstate, when the surface of the cathode 2 is irradiated with ultravioletradiation passing through the window member 4 and the mesh region 1m ofthe anode 1, photoelectrons are emitted from the cathode 2. Thusgenerated photoelectrons are accelerated toward the anode 1 due to theelectric field between the anode 1 and the cathode 2, and collide withmolecules of the gas between the anode 1 and the cathode 2, therebycausing an electron avalanche. Due to the electron avalanche, a numberof cations are generated between the anode 1 and the cathode 2. Thesecations are accelerated toward the cathode 2 by the electric field andcollide with the surface of the cathode 2, whereby a number of secondaryelectrons are emitted from the cathode 2. Like the photoelectrons, thesecondary electrons generate an electron avalanche, whereby thedischarge current between the anode 1 and the cathode 2 rapidlyincreases in response to incident ultraviolet radiation. Though thecharge of discharge current is supplied by a capacitor C1, the dischargeis terminated within a short period of time since the bias voltagebetween the anode 1 and the cathode 2 decreases in response to the rapidincrease in discharge current. Consequently, ultraviolet radiation aredetected as a current pulse. Generated at both ends of the resistor R2is a voltage pulse, which is monitored to detect ultraviolet radiation.During the fusion bonding, contaminants include fluorides and oxides areproduced on the surface of this partially assembled part. To removethese contaminants, a treatment using acid solution is performed.

In the following, a method of making the ultraviolet detector D1 shownin FIGS. 1 and 2 will be explained. First, the lead pin 6 is welded tothe lower surface of the cathode 2. Thus welded cathode 2 and lead pin 6are secured to the inside of the ring-shaped metal member (metal shell)5 by means of the glass sealant 7 that is fusion-bonded thereto. Thissecuring process is effected such that the upper surface of the cathode2 is placed at a predetermined height from the positioning surface 5a,and the metal evacuation pipe 8 is secured to the inside of thering-shaped metal member 5 by means of the glass sealant 7 such that theupper end of the metal evacuation pipe 8 projects above the positioningsurface 5a. The frequency at which pulses are generated is in proportionto the intensity of the ultraviolet radiation when the ultravioletradiation is low and saturated when the intensity of ultravioletradiation is high.

Subsequently, the lower surface of the lower end 1a of the anode 1 iswelded onto the positioning surface 5a. Accordingly, the mesh region 1mof the anode 1 and the upper surface of the cathode 2 are positioned onthe basis of the positioning surface 5a. Namely, the accuracy indistance between the anode 1 and the cathode 2 (i.e., discharging gap)is determined by the processing precision of the anode 1 and protrusionheight of the cathode 2 respect to the positioning surface 5a. Even whenthe cathode 2 connected to the lead pin 6 is somewhat deformed uponshock or heat, the distance between the anode 1 and the cathode 2 isheld with a high accuracy, thus reducing characteristic errors in eachultraviolet detector being produced.

Next, the window member 4 is fusion-bonded to the inside of the tubularmember 3 so as to close the upper opening of the tubular member 3 fromthe inside. And then, this partially assembled part is treated by acidsolution so that contaminants including fluorides and oxides areremoved. Thereafter, the tubular member 3 (cap) is mounted on thering-shaped metal member 5 such that the inner surface of the outwardcurved portion (flange) at the lower end of the tubular member 3 issuperposed on the outer surface of the outward curved portion (flange)at the lower end of the ring-shaped metal member 5, and these curvedportions are welded together. Since the tubular member is not made ofglass but a metal, fluorine which is contained in theultraviolet-transparent glass, for example by 1.9 wt % does not attachto the sealed vessel V1 even in this process. Also, since the tubularmember 3 is not made of glass, silica, which is a main component ofglass, does not evaporate upon this welding process, fine particles ofsilica are prevented from attaching to the sealed vessel V1 andelectrodes 1 and 2 and thereby causing abnormal discharge. Then, theevacuation pipe 8 is connected to a high vacuum apparatus so as toremove the gas from within the sealed vessel V1, and the sealed vesselV1 is externally heated so as to affect baking. After the pressurewithin the sealed vessel V1 is sufficiently lowered to attain asubstantially vacuum state, a reducing mixed gas is introduced into thesealed vessel V1 from the lower end of the metal evacuation pipe 8.After the gas is introduced, the lower end of the metal pipe 8 ispinched and sealed by pressure, thereby establishing a hermetic statewithin the sealed vessel V1. Since the metal evacuation pipe 8 is notmade of glass, even when one end thereof is thus sealed, fluorine andsilica are not introduced into the vessel V1.

In the following, an ultraviolet detector D2 in accordance with a secondembodiment of the present invention will be explained. FIG. 4 is a planview showing the ultraviolet detector D2. FIG. 5 is a sectional view ofthe ultraviolet detector D2 taken along line V—V of FIG. 4. Thisdetector differs from that shown in FIGS. 1 and 2 only in theconfigurations of the upper part of the tubular member 3 and the anode1. The diameter of the tubular member 3 differs between the upper partand lower part of the outer wall in its axial direction. Namely, theupper part of the outer wall has a diameter smaller than that of thelower part thereof, whereby their inner faces form a step 3s at theboundary therebetween. The step 3s of the inner face of the tubularmember 3 has a lower surface 3b in parallel with the window member 4.Welded to the lower surface 3b of the step 3s is the upper surface ofthe outer edge of the planar anode 1. The distance between the uppersurface 3c of the flange at the lower end of the ring-shaped metalmember 5 and the lower surface 3b of the step 3s is constant.Accordingly, the anode 1 is positioned with respect to the upper surface3c of the flange at the lower end of the ring-shaped metal member 5 whenthe anode 1 is simply welded to the lower surface 3b of the step 3s. Theupper surface of the cathode 2 is fixed by the glass sealant 7 such thatthe distance from the upper surface 3c of the flange is made constant.Accordingly, the distance between the mesh region 1m of the anode 1 andthe upper surface of the cathode 2 (i.e., discharging gap) is determinedon the basis of the upper surface 3c of the flange, and its accuracy isdetermined by the processing precision of step 3s of the tubular member3 and ring-shaped metal member 5. In the ultraviolet detector D2, afterthe anode 1 is fixed to the step 3s of the tubular member 3 whose oneopening is sealed with the window member 4, the tubular member 3 ismounted on the ring-shaped metal member 5 such that the inner surface ofthe outward curved portion (flange) at the lower end of the tubularmember 3 is superposed on the outer surface of the outward curvedportion (flange) at the lower end of the ring-shaped metal member 5, andthese curved portions are welded together, thus yielding the sealedvessel V1.

FIGS. 6 and 7 are vertical sectional views showing ultraviolet detectorsD3 and D4 in accordance with third and fourth embodiments of the presentinvention, respectively. The ultraviolet detectors D3 and D4 correspondto the ultraviolet detectors D1 and D2 shown in FIGS. 2 and 5,respectively, though differing therefrom only in that the evacuationpipe 8 is not provided. These detectors can be made by a methodcomprising the steps of introducing the tubular member 3 and thering-shaped metal member 5 which have not yet been welded together intoa vacuum chamber; heating the chamber; filling the chamber with a mixedgas; and then connecting these members to each other by resistancewelding technique.

FIGS. 8 and 9 are vertical sectional views showing ultraviolet detectorsD5 and D6 in accordance with fifth and sixth embodiments of the presentinvention, respectively. The ultraviolet detector D6 shown in FIG. 9 hasa configuration in which the evacuation pipe 8 is eliminated from theultraviolet detector D5 shown in FIG. 8. In the other respects, theirconfigurations are the same. The ultraviolet detector D5 differs fromthe ultraviolet detector D1 of the first embodiment in that the anode 1also serves as the tubular member 3. Due to such a configuration, itbecomes easier to manufacture a small detector in particular.

Finally, FIGS. 10 and 11 are vertical sectional views showingultraviolet detectors D7 and D8 in accordance with seventh and eighthembodiments of the present invention, respectively. The ultravioletdetector D8 shown in FIG. 11 has a configuration in which the evacuationpipe 8 is eliminated from the ultraviolet detector D7 shown in FIG. 10.In the other respects, their configurations are the same. Theultraviolet detector D7 differs from the ultraviolet detector D1 shownin FIG. 2 in the configuration of the anode 1. As compared with theultraviolet detector D1 shown in FIG. 2, the ultraviolet detector D7 maybe disadvantageous for keeping the distance between the anode 1 and thecathode 2 with a high accuracy. Nevertheless, due to its resultingsimpler configuration, it can be manufactured at a lower cost.

Without being restricted to the foregoing embodiment, the presentinvention can further be modified in various manners.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

The basic Japanese Applications No.8-255080 (255080/1996) filed on Sept.26, 1996 and No.8-270776 (270776/1996) filed on Oct. 14, 1996 are herebyincorporated by reference.

What is claimed is:
 1. An ultraviolet detector comprising: a sealedvessel including a tubular member, a window member and a stem, saidtubular member having an opening and being made of a metal materialblocking ultraviolet radiation, said window member being made of a glassmaterial transparent to ultraviolet radiation and closing said opening,said stem having a metal portion contacting to said tubular member and aglass portion not contacting said tubular member; an anodea cathodedisposed within said sealed vessel at positions opposing said windowmember; a cathode,an anode disposed within said sealed vessel betweensaid window member and said anodecathode, secured to said tubular memberor said metal portion of said stem; a lead pin penetrating said glassportion of said stem for securing said anode cathode and supplyingvoltage to said anode cathode; and a gas enclosed in said sealed vessel.2. An ultraviolet detector according to claim 1, wherein said cathodeanode is integrated with said tubular member.
 3. An ultraviolet detectoraccording to claim 1, wherein said metal portion is a ring shaped rim ofsaid stem.